Affibodies are relatively small proteins (compared to antibodies or antibody fragments) that can be radiolabeled and used to image specific targets in vivo. Dr. Dale Kiesewetter of NIBIB and Dr. Jacak Capala of NCI have synthesized F-18 labeled affibodies (F-18FBEM-Z-HER2:342 and F-18FBEM-Z HER2:2395) to image the HER2 receptor that is found in several types of cancer, including breast and lung. HER2 positivity is associated with resistance to certain types of treatment, so the ability to image the HER2 receptor in cancer patients could be useful in the selection of appropriate cancer treatment. In vitro and rodent studies with F-18FBEM-Z-HER2:342 have shown high affinity to the HER2 receptor, good clearance from blood, and high accumulation in HER2 positive tumors, suggesting it might be useful as a PET radiopharmaceutical in humans. Preclinical studies with F-18FBEM-Z-HER2:342 were begun in nonhuman primates to determine its biodistribution, clearance from blood and pathways of excretion, and to perform metabolite assays in blood. Preliminary data show rapid clearance of the tracer from blood, prominent urinary excretion, and retention of the tracer in kidney. Radiation dose estimates will be calculated. Radiation exposure to excretory pathway organs is important with regard to repeat imaging in humans, which could be required for clinical applications.